Application crashes automatically send telemetry back to Microsoft that developers can log in to their developer portal, and get hold of the stack trace that failed. Microsoft has said in the past that they concentrate on the most common failure points first, when talking about Windows 7 it was said:
The telemetry really helps us focus on the benefit of the change — fixing a bug that represents thousands of customers, a widely used device, or broadly used third party software has a much bigger impact than a bug that only a few people, lower volume device, or less used software product might address. With this data we can more precisely evaluate benefit of changes.
I would expect them to take a similar approach with their Windows Phone apps, as I would expect other third party developers.